The pathogenic chain of events in arthritic diseases is still poorly understood, but it is thought that different mechanisms converge on a common effector phase, which involves several key cells and molecules of the innate immune system. A considerable degree of genetic variation, in mice and human, conditions this inflammatory process. While some loci have been identified (IL-1, C5, Ncf1), most of this genetic variation remains uncharted. In order to best understand susceptibility on a population basis, there is a need for an integrated perspective that incorporates a representation, as complete as possible, of the species'genetic variation. This proposal builds on a successful R21 to dissect arthritis Quantitative Trait Loci (QTLs) in the K/BxN model via the potential offered by "Heterogenous Stock" (HS) mice. These animals were generated from 8 founder inbred strains by >50 generations'intercrossing;the accumulated recombination events bring the potential for high-precision QTL mapping, encompassing a broad representation of the species'allele pool. A screen involving 600 HS mice tested by transfer of K/BxN serum identified 33 QTLs at high resolution (3-8 Mb) and a high level of statistical significance (LOD score >10). This high number reflects the number of innate cellular and molecular pathways involved in the process. Identification of the genes underlying the 12 most prominent QTLs will be tackled. In Aim1, their position will be refined to the Mb range by denser genotyping around the QTLs and by testing additional mice from directed crosses. The causal genes will be pursued by a combination of sequence analysis and gene-expression profiling in founder inbreds and in RNA from a subset of the HS mice already screened. In Aim2, the strong Quantitative Trait Genes candidates (QTG) thus identified will be evaluated by using lentiviral vectors to create expression hypomorphs through RNA interference, and testing the resulting lines for susceptibility to arthritis. In Aim3, the human orthologs of the QTGs identified in the HS mice will be evaluated in RA patients: coding and promoter regions of the genes from 24 RA patients will be sequenced in order to identify potential disease-specific variants, and three cohorts of RA patients (2700 cases +ctls) will be genotyped for these and haplotype-tagging SNPs, searching for association to disease or to severity indices. Beyond identifying QTLs of relevance to arthritis, this project should provide valuable information on genetic variation in the innate immune system in general. This proposed project attempts to identify genes that control the severity and destructive consequences of inflammation in arthritis. To this end, we are mapping the genes that lead to strong disease in a strain of mice particularly informative for genetic analyses. Once identified, we will test whether these genes may also be involved in setting susceptibility to arthritis in human patients. These determinations should lead to a better understanding of the causes of arthritis, and suggest novel avenues for treatments.